Wind turbines are devices that convert mechanical wind energy to electrical energy. A typical wind turbine includes a nacelle mounted on a tower housing a drive train for transmitting the rotation of a rotor to an electric generator and other components such as a yaw drive which orientates the wind turbine, several actuators and sensors and a brake. The rotor supports a number of blades that capture the kinetic energy of the wind and cause the drive train rotational motion. The rotor blades have an aerodynamic shape such that when a wind blows across the surface of the blade, a lift force is generated causing the rotation of a shaft which is connected—directly or through a gearing arrangement—to the electrical generator located inside the nacelle. The amount of energy produced by wind turbines depends on the rotor blade sweeping surface that receives the action from the wind and consequently increasing the length of the blades leads normally to an increase of the power output of the wind turbine. The blades are controlled to stay in autorotation regime during normal phase, and its attitude depends on the wind intensity.
The dynamic coupling of the first symmetric in-plane mode of the 3 bladed rotor with the drive-train main frequency results in a coupled mode which is practically undamped in the wind turbine operation. This coupled mode may even be excited when operating at nominal power for high wind speeds leading to unaffordable loading on the drive-train. A wind turbine control operation without considering such dynamics can easily lead to damaging levels of fatigue loading on the gearbox.
The prior art teaches the use of the generator torque reference for damping said vibrations. This technique is highly dependant of a good identification of the drive train main frequency vibrations.
US 2006/0066111 discloses a vibration damping technique for variable speed wind turbines that not only aids damping of drive train vibrations caused by variation in wind speed, but also mitigates tower loads caused by side-to-side oscillations of the tower. Further, the technique advantageously reduces power fluctuations of the generator coupled to the wind turbine rotor. Said vibrations are determined as a function of the rotor speed using Fourier transforms in real-time operation.
A drawback of said proposal regarding particularly to the identification of the drive train vibrations is that the Fourier transforms require time windows of data of a certain size that may cause important delays in the processing of the generator speed signal.
The present invention focuses on finding a solution for said drawback.